The present invention relates to a method for the nondestructive inspection, by ultrasound, of materials and, more particularly, of welds joining two metal workpieces together, edge to edge, in particular of the plate or sheet kind.
This method makes it possible to inspect, during manufacture and in service, structures and equipment comprising welds, for example those obtained by local melting, which joints may contain plane flaws of the crack kind or volume flaws; in this regard, the terms “flaw” and “crack” will be used below without distinction.
As non-limiting examples, the method according to the invention may apply to the inspection of the welds of the walls of equipment or components operating under pressure, forming part of chemical or petrochemical units, nuclear power stations or the like, structures of rotating machines, elements or components of rotating machines, or pipes, in situations that may require inspections, and especially repeated examinations, such as examinations before and after a heat treatment, or before and after a repair, the monitoring, in use, of materials or assemblies, the monitoring of any change in flaws in the pressurized equipment (such as cracks changing in service), industrial quality control in manufacturing, etc.
One particular technique called TOFD (time-of-flight diffraction), applicable to ultrasonic inspection, was developed by M Silk from 1973 onwards and is described, for example, in British Standard BS 7706: 1993 or in the work by J P Charlesworth and JAG Temple “Engineering Applications of Ultrasonic Time-of-Flight Diffraction” (1989–2001).
However, no document has indicated, hitherto, means for effective detection, by simple echography, random flaws located in a weld volume at between 5 mm and 100 mm in depth.
This is because the main difficulties presently encountered during an ultrasonic examination using the TOFD technique, which are awaiting industrial solutions, are the following:                in the case for example of a pair of ultrasonic transducers in contact with a non-plane surface of the component to be examined, it is generally necessary to change transducer for complete examination of a joint between walls having thicknesses of between 5 mm and 100 mm;        actual cracks are not always detected with the conventional procedures, mentioning ranges of transducers or of refraction angles that are too blurred;        it is also necessary to change a pair of transducers in order to use different distances between points of emergence and fraction angles adapted to the various thicknesses, such as for example when searching for flaws that may be located near the surface of the weld or, on the contrary, close to the wall on the opposite side from the weld surface; and        when searching for flaws of the crack type, no rating criterion exists that can ensure 100% detection of emergent cracks or of internal cracks likely to emerge on the internal or external surface, and the correlation of which, with a radiographic examination, has demonstrated better performance than the ultrasonic technique.        
To try to solve these problems, a number of solutions have already been proposed.
Thus, document GB-A-2 198 532 has proposed an inspection method of the TOFD (time-of-flight diffraction) type for welded objects under water, such as oil extraction platforms, using two transducers adjacent the surface of the object to be inspected and spaced apart, one on either side of the weld joint, in which any cracks have to be detected.
However, the solutions proposed up until now have proved unsatisfactory, insufficient or imperfect from the industrial standpoint, especially as they do not allow effective detection of cracks in the welds.
The present invention aims to solve the problems existing in the prior art by proposing an improved method of inspecting welds that effectively covers at least the depth range between 5 mm and at least 60 mm, in a single pass along the weld to be inspected, whether along the longitudinal direction or the circumferential direction, i.e. for linear welds or circular welds (butt welding of tubes), and also the smaller depth range, namely the regions close to the surface at a depth of between 0.5 mm and 5 mm, which are usually more difficult to inspect.
In addition, the invention also aims to allow auscultation of a larger thickness or volume region using a reasonable number of transducers.
Finally, the object of this invention is also to provide a simple and autonomous apparatus suitable for implementing the inspection method in question from an industrial standpoint.
The solution of the present invention is therefore a method for ultrasonic inspection of welds that combines the TOFD technique with the CW (creeping wave) technique.